The absorbent material used in many control rods for LWR's and FBR's is boron carbide (B.sub.4 C). Boron carbide has a high cross section for absorption of neutrons and maintains its properties at reactor operating temperature. It is the present practice to form boron carbide either into cylindrical pellets that are subsequently loaded into cladding tubes, or it is loaded into similar cladding tubes in powder form by vibro-compaction techniques. Recent experience in a boiling water reactor has revealed the key limitation to the mechanical integrity of these control rods, the swelling behavior of the boron carbide. Neutrons absorbed on the outer diameter of the boron carbide material cause a volumetric increase in the boron carbide and a growth of the pellet, or compacted powder, in an arrangement which bears upon and stresses the metal cladding (usually stainless steel or inconel). Dimensional changes have warped the tubes or even ruptured the cladding tube which leads to a loss of function for the control rods.
An internal structural arrangement of boron carbide in the cladding tube must be provided to reduce stresses on the cladding tube by the dimensional expansion of the boron carbide.